A high impact polystyrene resin (HIPS) is a polystyrene resin having incorporated therein a rubbery polymer such as a conjugated diene polymer, which exhibits improved impact resistance. As the high impact polystyrene resin has good impact resistance, transparency, luster, rigidity and shapability, and is inexpensive, it is widely used for electrical equipments, office appliances, packaging containers and sundry goods.
A polybutadiene rubber is most popularly used as a resin-modifier for the preparation of the high impact polystyrene resin. To more improve the impact resistance, transparency and luster of the high impact polystyrene resin, the incorporation of block copolymers have been proposed in, for example, Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) S63-48317, JP-A S64-74208, JP-A H1-172413, JP-A 2002-179715 and JP-A 2003-48926.
The block copolymers proposed in these patent publications easily become powdery and have poor handling characteristics, and are difficult to be shaped into, for example, bales. To solve these problems, a resin-modifying composition comprising an aromatic vinyl-conjugated diene block copolymer, the block aromatic vinyl portion of which has a relatively small molecular weight, and which has as the whole a relatively large molecular weight and a relatively narrow molecular weight distribution, has been proposed in JP-A H10-204136. A rubbery polymer composition comprising an aromatic vinyl-conjugated diene block copolymer and a conjugated diene homopolymer, which has been made by a special process, has been proposed as a resin-modifier in JP-A H5-331247. A radial block copolymer comprising a combination of two kinds of polymer segments having specific glass transition temperatures is known as a resin-modifier (see JP-A S64-79251).